When there are huge explosions on the Sun you don't know immediately if it has an Earth-directed Coronal Mass Ejection (CME) associated with it. In this article we dig a bit deeper in the methods that you can use so that you can determine when we have an Earth directed CME and when it's going to arrive on Earth.
On our website you'll find a data plot with the primary >10MeV solar protons, or in short the Proton flux monitor. During huge explosions on the Sun a solar radiation storm can be triggered. The solar protons get blown into space near the speed of light and are the first particles to arrive at Earth. So we know very fast if a solar radiation storm is triggered and, in a less accurate way, if there is a CME directed to Earth.
Solar radiation storm
The best way to be sure that a CME is Earth-directed are with the help of images taken by the SOHO spacecraft (LASCO C2 and C3 instruments) and the STEREO spacecraft. You'll find the latest images and videos on our website.
On the LASCO images you can immediately see if the CME is Earth directed or not. A CME is always good visible in the imagery and when it's Earth direct you'll see a full or partial halo CME. So this means it is partial or fully Earth-directed.
With the use of the imagery available from the spacecrafts the space weather scientists calculate the departure speed and the Estimated Time of Arrival (ETA) of the CME. If you want to know the CME could arrive you will have to wait on reports from the space weather scientists. The daily reports are also available on our website.
Keep in mind that the LASCO images are not always in real-time and you'll might have to wait several hours for new images and to know if it's Earth directed or not.
Example of a full halo CME
The Belgian Solar Influences Data Analysis Center (SIDC) developed the CACTUS program which stands for Computer Aided CME Tracking. It automatically scans imagery from LASCO to determine whether a CME is likely to hit Earth or not. It will show if a CME is a halo CME or partial halo CME and also determines the lift off speed of the CME. It can take a while before the LASCO images are processed, most LASCO images become available after about 6 hours.
EPAM stands for the Electron, Proton and Alpha Monitor and is an instrument on the ACE satellite that measures the electrons and protons that are send out with the solar wind. It's a very useful instrument to know if a CME is Earth directed and when it's going to arrive. We'll be making it a bit clear with some EPAM plots.
The EPAM plot, just after a solar flare
When a huge explosion happens on the Sun, electrons and protons are hurled away from the Sun into space. The electrons and protons are pushed out with the solar wind flow. Immediately after such an event the EPAM plot shows a fast rise of the electrons which marks the start of the flare. The proton plot will also show a steady rise. This indicates that a part of the CME is Earth directed. Now we know that a CME shock front is on it's way to Earth but one critical question remains: when is it going to arrive?
If we know the speed of the CME, we can easily determine when it is going to arrive. With the following table we listed for different CME speed what the average travel time of a CME is. But keep in mind that this is not an exact travel time. It is common for a CME to arrive later or earlier then the predicted arrival time with a margin of sometimes 6 hours.
|speed CME km/s||travel time (hr)||days||hours|
After the solar flare, the EPAM proton plot will keep rising until the arrival of the CME shock. The first rise in the plot is called the "ramp up" or "onset" phase. It keeps rising but flattens when the CME gets closer. A few hours before the actual arrival of the CME a new rise will take place, this indicate that the shock front is going to arrive soon. When the EPAM plot peaks, then it indicates that the CME has arrived. After the peak the proton level will decline to normal values unless there is another Coronal Mass Ejection on on it's way to Earth.
On the EPAM plot below you can see this very good. We can see a clear rise in all values after the solar flare with a good ramp up phase followed by the flattening of the values before rising again to indicate the imminent arrival of the CME. At the peak, the shock wave arrived.
The EPAM plot
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